Light-emitting diodes (LEDs) are used as a kind of solid-state light source. Compared with traditional light sources, such as incandescent or fluorescent lamps, its advantages are compactness, high efficacy, good color, various and variable colors, etc. Thus, LEDs are widely applied in indoor lighting, decoration lighting, and outdoor lighting. Some of these applications require the output light from the LEDs to be adjustable from 1% to 100% of the maximum light output, that is, users often require a dimming capability.
In order to dim the light output of the LEDs, it is required to control the output current of the LED driver to follow a certain dim input. Currently, most LED drivers achieve the dimming function by chopping the output current through an extra Mosfet, and the current to the LEDs can be controlled by changing the duty cycle of the Mosfet via a dim input. Alternatively, the dimming function is achieved by modulating the output current by a dim input, which is usually an analog voltage level or PWM (pulse width modulation) signal. These dimming methods have a common feature in that the dim input is at the secondary side of the driver, which is referred to as secondary dimming.
In traditional lighting, a phase-modulating dimmer is commonly used for dimming the light output and is usually connected at the power input terminal of the driver. The phase-modulating dimmer cuts the phase of the input voltage from the power supply, and finally the output current to a burner is controlled. By turning a knob of the dimmer, users can thus easily control the light output. Since the dim input is at the primary side of the driver, such a dimming method is referred to as primary dimming.
Due to the dim input of the LED driver described above at the secondary side rather than at the primary side, these LED drivers are incompatible with phase-modulating dimmers, which are originally utilized to alter the brightness or intensity of the light output in traditional lighting. Consequently, many of these drivers are incompatible with the existing lighting system infrastructure, such as the lighting systems typically utilized for incandescent or fluorescent lighting.
It is therefore desirable to develop a LED driver which is compatible with the existing phase-modulating dimmers.